.I create a morph to control the tread movement, in scene mode I go to the morph in the timeline click on the applicable morph which gives me the key value editor turn on expression, click edit then add the script, I exit the editor and the treads move as predicted.

I then save the scene. However, when i return again at a later point. the script is no longer in the expression editor it has not saved it. this happens with most of the newer beta versions from v1.00 to build 1314